The Banc Populaire and Hydroptere are completely different craft. Hydroptere uses a development of Bruce foils; where leeward foil gives lift and windward foil pulls back into water to counteract heeling. If you look at foil on BP its a dagger is 90 degrees to hull whereas foil on Hydroprtere is pointing back in intowards boat.(45 degrees?)(Leaning in that way means that because of leeway (under sail) it "pulls" hull into water.(when its windward foil, reverses on other tack) When flying its balanced on rudder (foiled) leeward foil and heeling force of sail. (Hydroptere also fills its windward "float" with water)